1. Field of the Invention
This invention relates to the field of well logging and, more particularly, to well logging methods and apparatus for determining formation properties, such as conductivity, in high-contrast thin-layer formations or at high dip angles, with greater accuracy than prior methods. Still more particularly, this invention relates to an improved method for processing multiple voltage data obtained during logging with an induction array well tool and creating a depth tagged representation of resistivity or conductivity from a formulated difference of the voltage data. The invention has general application in the well logging art, but is particularly useful at a well site while logging.
2. Description of Related Art
Induction logging is a well-known form of electromagnetic logging used to make a determination of the conductivity (or its inverse, resistivity) profile of the earth formations surrounding a borehole. U.S. Pat. Nos. 3,340,464; 3,147,429; 3,179,879; 3,056,917; and 4,472,684 are illustrative of typical well logging tools that utilize the basic principles of induction logging.
Conventional induction logging tools or "sondes" include a transmitter and a receiver array consisting of a set of coils mounted on a support and axially spaced from each other in the direction of the borehole. The transmitting coil is energized by an alternating current, which in turn generates an electric field that induces eddy currents in the formation surrounding the borehole. The intensity of the eddy currents is proportional to the conductivity of the formation. The field generated in turn by these eddy currents induces an electromotive force in the receiving coil. Phase-locked detection, amplification, and digitization of this signal determines the amplitude and phase of the receiver voltage, usually expressed as a complex number (phasor voltage). By processing the receiver coil voltages, an evaluation of the formation conductivity profile is obtained. U.S. Pat. No. 5,157,605 (assigned to the present assignee) discloses an induction array well tool that may be used to develop voltage data for processing by the present invention.
Conventional techniques for evaluating the conductivity of formations have practical limitations. Neighboring layers in high-contrast (in terms of resistivity) thin-layer formations can corrupt the measurement results--known as shoulder effect. At high dip angles, horns and other artifacts are seen in the resistivity logs. Modeling and actual measurements have confirmed these effects. See B. Anderson et al., Response of 2-MHz LWD Resistivity and Wireline Induction Tools in Dipping Beds and Laminated Formations, SPWLA THIRTY-FIRST ANNUAL LOGGING SYMPOSIUM, Pp. 1-25, 1990. The cause of the horns is transverse magnetic ("TM") coupling, which becomes important at high dip angles. The TM horns observed are useful to detect layer boundaries, but are detrimental for quantitative formation evaluation.
U.S. Pat. No. 5,041,975 describes a method for correcting data developed by the well tool of the '605 patent, to eliminate the effects of the borehole on the measured data. U.S. Pat. No. 5,184,079 describes a method for correcting logs, developed from a logging tool, disposed in a wellbore at a dip angle relative to the formation layers, to eliminate the effects of the dip angle on the resistivity logs. The applicability of this correction method is limited to dip angles less than about 45 degrees. Another method for correcting induction logs with high apparent dip angle effects was described by Barber et al., Interpretation of Multiarray Induction Logs in Invaded Formations at High Relative Dip Angles, SPWLA THIRTY-NINTH ANNUAL LOGGING SYMPOSIUM, June, 1998. This method requires time-consuming model calculations, making it less desirable for operations at the well site while logging. An underlying criterion of the methods described by the '079 patent and Barber et al. is the requirement that the dip angle be known before the methods can be applied.
It is desirable to obtain a simplified method of processing data, acquired from a well tool, to correct for dip effect and neighboring layers in high-contrast thin-layer formations. Still further, it is desired to implement a data processing technique that is not restricted to piecewise constant formations and does not require prior knowledge of dip angles. Thus, there remains a need for a simplified process and apparatus that produces accurate conductivity profiles of unrestricted formations from data developed by a well tool.